Even by the wildest of Bohemian standards, it was a complex menage. Artist Augustus John was driven by an insatiable sexual appetite that nearly destroyed the lives of the women who loved him best. His domestic life was an extraordinary tangle of passions. His wife, Ida, although at first jealous of his beautiful mistress, Dorelia, eventually fell in love with her. As did Augustus's sister Gwen.

Small wonder that John's reputation - in his youth he was feted as the most famous British artist of his day - has been overwhelmed by prurient fascination with his sexual life. As art critic Brian Sewell put it so succinctly, he was driven 'to draw the women whom he bedded, and bed the women whom he drew.'
His sister, Gwen, on the other hand, who had almost disappeared from history, was rediscovered by female historians - who lauded her as a great woman artist who could have painted a refrigerator water filter and made it a great piece of art, though she had always been overshadowed by her brother.
Now, however, a new exhibition at Tate Britain (which open on 29th September 2004) - the first large-scale show to compare the work of the brother and sister - will prove that Augustus deserves to regain his former standing as the man who could 'draw like a god'. Born in 1878, Augustus was two years younger than Gwen. The John family lived in Haverfordshire in Pembrokeshire, later moving to Tenby. Their mother died when the children were very young; their father, Edwin, a solicitor, was stern and cold. And yet the children enjoyed remarkable freedom, running wild on the lovely Welsh beaches.

Augustus was shy and awkward, even when he went to London to study art at the Slade. But the summer he turned 18 he changed dramatically, after cracking his skull when diving. Legend has it that he banged his head on a rock - and emerged a genius. Nonsence - but it was the start of the Augustus John myth, of the artist as the great Bohemian and lover. His drawings, including beautiful nudes, had a new, spontaneous assurance. He was impatient, restless, unpredictable. An imposing, scruffy, red-bearded figure, six feet tall, who shockingly wore gipsy earrings, he became renowned partly because, in late-Victorian London, he stood out so. 'We are the kind of people our fathers warned us against,' he boasted to a fellow student.

It was at the Slade - where his sister Gwen was also studying - that Augustus met and fell in love with Isa Nettleship. Sensuously beautiful, in a Pre-Raphaelite way, she had almond eyes, a mass of dark hair and full lips. She was exhilarated by Augustus, the only man she ever loved. In 1901, Augustus eloped with her and they were married - motherhood caught up with her, and she lost the fey, mysterious quality that attracted him.

Augustus desired an ideal mistress and earht mother in one. He spun all his romantic fantasies around Dorelia McNeill - his muse, his lover, the woman who came to enthral his wife and his sister, too. She was in fact a young typist called Dorothy, who was taking drawing lessons from Gwen. By 1903, Augustus was writing to Dorelia passionately: 'I am sick for love of you... Yes, I possess you as you possess me.' It wasn't just her sultry beauty; what people remarked upon was Dorelia's calm, soothing serenity.

Augustus introduced her to poor Ida who, despite herself, found that she actually liked Dorelia. And hoping that a menage a trois would save her marriage, she agreed that they should find a big house where they could all live together. Dorelia, however, decided to 'elope' with Augustus's sister, Gwen, and proposed that she and Gwen should walk to Rome. They hitched lifts and slept in haystacks, living intimately together - but it was tough going and they never made it to Rome. They turned back and headed to Paris.

Gwen painted several sensitive portraits of Dorelia, some of the finest work she ever did. Augustus was eaten up with jealously when he heard his mistress was posing for other artists in the nude, as she had never agreed to do so for him, and set off for Paris to ceck up on her - only to find that Dorelia had fled to Belgium, in the arms of another man, a young artist called Leonard.

All three of the Johns were distraught. Ida was determined to persuade her husband's mistress to return to him: 'By God, I will haunt her until she comes back,' she said. But it was Gwen who did all the begging: 'Dorelia, you know I love you,' she appealed. Gwen's forcefulness could be terrifying. Dorelia gave in.

Meanwhile, another chapter opened in Gwen's life. That summer of 1904, aged 27, she had fallen ecstatically in love with the 63-year-old sculptor Auguste Rodin. Unfortunately, her obsessive love soo wore him out. He complained that sex gave him a headache and once a fortnight was enough; he felt devoured by her. When Rodin became involved with someone else, Gwen Rodin died in 1917.

Back in England, life a trois didn't prove easy. Isa's domestic situation cut her off from friends and family. She wrote snappishly to a curious woman friend, telling her 'not to bother me any more to know "where Domelia sleeps". You know we are not a conventional family.' By now Domelia had given birth to a child and motherhood brought the two women closer. Like Gwen, Ida fell under Domelia's spell. When they were parted for a few days she wrot, 'I was bitter cold last night without your burning hot, not to say, scalding, body next to me.' Now Ida proposed they should 'elope' and set up home in Paris, where Augustus could visit when it suited him.

Their life in Paris, with only one bed for the three of them - the children slept in boxes - was a bit too chaotically Bohemian even for Augustus. Left to his own devices in London, he found himself a new mistress. Eventually, Dorelia moved into lodgings of her own with her children. Isa was horrified, however, when Augustus decided he wanted to live with her again but felt she had to agree. Besides, she was pregnant yet again. However, within a week of giving birth to her fifth child in 1907, Ida was dead of puerperal fever. She was 29. Three months later, Augustus was deep in another passionate affair and Dorelia had come back to look after the children.

Augustus John died in 1961. He was a monster of selfishness. But so was his sister. She became a recluse, turning to religion after Rodin's death. When she died in Dieppe in 1939, Gwen was buried in a pauper's unmarked grave. And yet Augustus maintained, ' Gwen and I were not opposites but much the same really. She wasn't chaste and subdued but amorous and proud...Her passions for both men and women were outrageous.' Gwen was a shadowy spinster to the world she scorned, but not to the brother who saw her as the mirror image of himself.